The present invention relates to an ion source assembly for calutrons and more particularly to an improved assembly that provides for the efficient separation of elements having high vapor pressures while at the same time permitting operation of the source assembly at lower temperatures and providing reliable control of the vapor flow from the charge bottle to the arc chamber. It is the result of a contract with the U.S. Department of Energy.
The calutron (electromagnetic) separation of isotopes begins with the vaporization of the element to be separated. Ideally, the vapor is supplied to the ionization chamber at a controlled rate so that optimum operating conditions can be established and maintained. This is accomplished by applying heat to a charge bottle containing the material to be separated and allowing the vaporized material to pass into the ionization chamber. The amount of heat to be applied is determined by the vapor pressure of the material to be vaporized and must be controlled accordingly. Excessive heat results in excessive vapor being discharged into the ionization chamber and electrode area. This has a detrimental effect on the equipment and the amount and purity of the product, as previously described in U.S. Pat. No. 3,098,123. Control of the heat and subsequent control of the vapor is particularly critical in the separation of an element (e.g., mercury) having a high vapor pressure and requiring a low operating temperature.
Difficulties encountered in previous separations of mercury created a need to improve the heat and vapor control in the ion source utilized in such separations. In prior ion sources, the heat generated within the equipment and acting on the charge bottle contributed to the problem of overheating. This unpredictable and uncontrollable heat from source drains, electrode drains, arc power and filament power preempts the applied heat causing erratic fluctuations in the vaporization pattern. Previous attempts to correct the problem for mercury isotope separation are described below.
1. A cast copper holder for the charge bottle equipped with a cooling coil for circulating cooling water was used to overcome the extraneous heat effect. This was mildly successful, but the time lag involved in this dual control caused over-shooting or under-shooting of the heaters. The copper was subject to corrosion and the unit was difficult to clean and service.
2. A vapor valve between the charge bottle and the ionization chamber was tried but was unsuccessful in controlling the vapor flow because of plugging.
3. Mercury metal was tried as an external charge, but the problem of adequately heating the supply line was never satisfactorily solved.
4. A method is described in U.S. Pat. No. 3,700,892 in which an internal mercury charge was heated by circulating water at a temperature of 100.degree. C. through a liner adjacent to the charge bottle. The effect of extraneous heat was eliminated, but the proper mercury charge vapor for optimum source operation was never reached with the heat supply limited to the 100.degree. C. water. Another objectional feature of this system was the safety hazard involved in manipulating the hot water line connections.
5. The most recent mercury separation was accomplished by using HgCl as a charge feed. This feed was used to obtain some increase in ion output and to eliminate the safety hazard that was present in the hot water heated system. Extraneous heat was again a problem producing unstable performance. The greatest difficulty was caused by the release into the system of HgCl and Cl.sub.2 gas. These caused high background pressures and required calcium pumping to lower the pressure. By the chemical action of the Ca with HgCl and Cl.sub.2, hydroscopic CaCl.sub.2 was deposited on surfaces of the equipment. When the system was let down to air, the Ca and CaCl.sub.2 reacted with H.sub.2 O in the air and large amounts of the H.sub.2 O were retained on the surfaces of the source, receiver, liner and manifold. It was necessary to remove this residual moisture by outgassing before another run could be started. Complete outgassing of a unit required 20-60 hours and in many cases the liner receiver and the source components had to be washed before acceptable operating pressure could be attained. The cost of operating a calutron is approximately $30.00 per tank hour and this pump-down or outgassing time adds to the cost of the isotopes which is eventually borne by the users of the isotopes.
It is the primary object of the present invention to provide an improved ion source assembly for calutrons for achieving an efficient separation of elements having high vapor pressures, for permitting operation of the source at lower temperatures, and for achieving reliable control of the vapor flow in the source to provide optimum source operation.
Other objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description of a preferred embodiment of the invention and the accompanying drawings.